in the CMB have confirmed the predictions of big bang nucleosynthesis (BBN) that the dark matter must be nonbaryonic. Gravitational lensing measurements have directly mapped its large-scale distribution, and the combination of lensing and X-ray measurements has severely challenged many of the modified-gravity alternatives to dark matter.

• Supernova data, along with other cosmological observations, imply that the expansion of the universe is accelerating. This surprising result suggests either a breakdown of general relativity on the scale of the observable universe or the existence of a novel form of “dark energy” that fills space, exerts repulsive gravity, and dominates the energy density of the cosmos.

• The discovery that neutrinos oscillate between their electron, muon, and tau flavors as they travel, and hence that they have mass, provides evidence for new physics beyond the standard model of particle physics. The effects of oscillations were seen in the first experiment to measure solar neutrinos, and the interpretation was confirmed by measurements of atmospheric neutrinos produced by cosmic rays and by new solar neutrino experiments with flavor sensitivity.

• In the past few years, a cutoff has been seen in the energy spectrum of ultrahigh-energy cosmic rays (UHECRs) consistent with that predicted to arise from interactions with the CMB. UHECRs have become a powerful tool for probing the active galactic nuclei (AGN), galaxy clusters, or radio sources responsible for accelerating such particles.

Looking forward to the coming decade, scientists anticipate further advances that build on these results.

The Astro2010 Science Frontiers Panel on Cosmology and Fundamental Physics was tasked to identify and articulate the scientific themes that will define the frontier in cosmology and fundamental physics (CFP) research in the 2010-2020 decade. The scope of this panel report encompasses cosmology and fundamental physics, including the early universe; the cosmic microwave background; linear probes of large-scale structure using galaxies, intergalactic gas, and gravitational lensing; the determination of cosmological parameters; dark matter; dark energy; tests of gravity; astronomical measurements of physical constants; and fundamental physics derived from astronomical messengers such as neutrinos, gamma rays, and ultrahigh-energy cosmic rays.

In response to its charge, the panel identified four central questions that are ripe for answering and one general area in which there is unusual discovery potential:

• How did the universe begin?

• Why is the universe accelerating?

• What is dark matter?

• What are the properties of neutrinos?

• Discovery area: Gravitational wave astronomy.